Stent With Improved Flexibility

ABSTRACT

A stent includes a continuous wave form wrapped around a longitudinal axis of the stent at a pitch to define a helix comprising a plurality of turns. The wave form includes a plurality of struts and a plurality of crowns. Each crown connects adjacent struts within a turn to define the continuous wave form. The stent also includes a plurality of connections configured to connect selected crowns of adjacent turns. Unconnected crowns of adjacent turns that substantially face each other are spaced from each other and define a gap therebetween. The gap between the unconnected crowns of adjacent turns is variable around a circumference of the stent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 61/243,592, filed on Sep. 18, 2009, theentire content of which is incorporated herein by reference. Thisapplication also claims the benefit of priority from U.S. ProvisionalPatent Application Ser. Nos. 61/243,578, 61/243,581, 61/243,582,61/243,597, and 61/243,600, all filed on Sep. 18, 2009, the entirecontents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to a stent having improvedflexibility along the length of the stent, and a method formanufacturing a stent having improved flexibility along the length ofthe stent.

2. Background of the Invention

A stent is typically a hollow, generally cylindrical device that isdeployed in a body lumen from a radially contracted configuration into aradially expanded configuration, which allows it to contact and supporta vessel wall. A plastically deformable stent can be implanted during anangioplasty procedure by using a delivery system that includes a ballooncatheter bearing a compressed or “crimped” stent, which has been loadedonto the balloon. The stent radially expands as the balloon is inflated,forcing the stent into contact with the body lumen, thereby forming asupport for the vessel wall. Deployment is effected after the stent hasbeen introduced percutaneously, transported transluminally, and trackedand positioned at a desired location by means of the balloon catheter.

Stents may be formed from wire(s), may be cut from a tube, or may be cutfrom a sheet of material and then rolled into a tube-like structure.While some stents may include a plurality of connected rings that aresubstantially parallel to each other and are oriented substantiallyperpendicular to a longitudinal axis of the stent, others may include ahelical coil that is wrapped around the longitudinal axis at anon-perpendicular angle.

SUMMARY OF THE INVENTION

It is desirable to provide a stent that is flexible to minimize thetracking effort through tortuous vessel anatomy, and a stent that isconformable to the vessel wall, yet provides adequate radial strength tosupport the vessel.

In an embodiment of the present invention, a stent includes a continuouswave form wrapped around a longitudinal axis of the stent at a pitchangle to define a helix comprising a plurality of turns. The wave formincludes a plurality of struts and a plurality of crowns. Each crownconnects adjacent struts within a turn to define the continuous waveform. The stent also includes a plurality of connections configured toconnect selected crowns of adjacent turns. Unconnected crowns ofadjacent turns that substantially face each other are spaced from eachother and define a gap therebetween. The gap between the unconnectedcrowns of adjacent turns is variable around a circumference of thestent.

In an embodiment of the present invention, there is provided a method ofmanufacturing a stent. The method includes forming a wave formcomprising a plurality of struts and a plurality of crowns. Each crownconnects adjacent struts. The method also includes wrapping the waveform around a longitudinal axis at a pitch angle relative to thelongitudinal axis to define a helix that includes a plurality of turnssubstantially centered about the longitudinal axis, connecting selectedcrowns of adjacent turns, and forming a variable gap between unconnectedcrowns of adjacent turns that substantially face each other around acircumference of the stent along the pitch angle.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a conventional stent;

FIG. 2 schematically depicts a stent in accordance with an embodiment ofthe present invention;

FIG. 3 schematically depicts a stent in accordance with an embodiment ofthe present invention in an unrolled condition;

FIG. 4 is a more detailed view of a portion of the stent of FIG. 3;

FIG. 5 schematically depicts a portion of a stent in accordance with anembodiment of the present invention;

FIG. 6 schematically depicts a portion of a stent in accordance with anembodiment of the present invention;

FIG. 7 schematically depicts a portion of a stent in accordance with anembodiment of the present invention; and

FIG. 8 schematically depicts a portion of a stent in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and use of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

FIG. 1 illustrates a conventional stent 10 that includes a plurality ofturns 20. Each turn 20 includes a plurality of struts 22 and a pluralityof crowns 24. Each crown 24 connects two adjacent struts 22 within aturn 20. The turns 20 may be formed individually as rings, and then maybe connected together with connections 30 that connect selected crowns24 of adjacent turns 20 of the stent 10, as illustrated in FIG. 1. Theturns 20 are placed along a common longitudinal axis LA so that thecrowns 24 of adjacent turns 20 that face each other either contact eachother or almost contact each other, i.e., a spacing between the adjacentcrowns may be less than about 0.001″. As illustrated, the turns 20 aresubstantially parallel to each other and are oriented substantiallyperpendicular to the longitudinal axis LA of the stent 10.

When the stent 10 of FIG. 1 is crimped onto a delivery system, such as aballoon catheter, and tracked through curved lumens, the stent 10substantially conforms with the curves in the lumens by bending. Thecrowns 24 have a tendency to rub against each other and even pushagainst each other, thereby restricting movement of the unconnectedcrowns 24 and decreasing flexibility along the length of the stent 10.

To overcome the decrease in flexibility and yet still track the stent 10through a curve in the lumen, the operator typically increases theamount of force applied to the delivery system so that an additionalamount of force may be applied to the interfering crowns 24 to generallyconform the interfering crowns with the curve in the lumen.

FIG. 2 illustrates a stent 110 in accordance with an embodiment of thepresent invention. As illustrated, the stent 110 includes a plurality ofturns 120. Each turn 120 includes a plurality of struts 122 and aplurality of crowns 124 or turns. Each crown 124 connects two adjacentstruts 122 within a turn 120. The turns 120 may be formed individuallyas rings, and then may be connected together with connections 130 thatconnect selected crowns of adjacent turns 120, as illustrated in FIG. 2.

The connections 130 may be created by fusing the selected crowns 124together. As used herein, “fusing” is defined as heating the targetportions of materials to be fused together, without adding anyadditional material, to a level where the material in the targetportions flow together, intermix with one another, and form a fusionwhen the materials cool down to, for example, room temperature. Asuitable laser may be used to create the fusion.

In an embodiment, the connections 130 may be created by welding orsoldering the selected crowns 124 together. As used herein, “welding”and “soldering” are defined as heating an additional material that isseparate from the selected crowns and applying the heated additionalmaterial to the selected crowns 124 so that when the additional materialcools, the selected crowns 124 are welded or soldered together.

In an embodiment, the connections 130 may be created by fusing, welding,or soldering an additional piece of material (not shown) that extendsbetween selected crowns 124. The additional piece of material mayresemble a strut or a portion of a strut, and may be sized to providespacing between the selected crowns of two adjacent turns, if desired.In an embodiment, the stent 110 may be cut from a tube, and theconnections 130 may include material from the tube. The illustratedembodiments are not intended to be limiting in any way.

In contrast to the stent 10 of FIG. 1, the turns 120 of the stent 110 ofFIG. 2 are placed along a common longitudinal axis LA so that the crowns124 of adjacent turns 120 that face each other, but are not connected,are spaced from one another to create a gap 140. In an embodiment, thegap 140 may be in the range of between 0.001″ and 0.003″. The size ofthe gap 140 can vary based on the physical characteristics of the stent,such as strut length, crown design, stent diameter, strut diameter, andother characteristics. In an embodiment, the gap 140 may be in the rangefrom just above zero, i.e. greater than zero, to as high as a distancethat is equal to the length of the longest strut, i.e., less than orequal to the length of the longest strut. In an embodiment, the gap 140may be in the range of between about 0.0005″ and about 0.010″.

It has been found that the stent 110 illustrated in FIG. 2 is generallymore flexible than the stent 10 illustrated in FIG. 1, because theunconnected crowns 124 of the adjacent turns 120 that face each other donot interfere with each other when the stent 110 is flexed or bentrelative to the longitudinal axis LA.

FIG. 3 illustrates a stent 210 according to an embodiment of the presentinvention. The stent 210 is generally cylindrical in shape and has alongitudinal axis LA extending through the center of the stent 210. FIG.3 illustrates the stent 210 in an “unrolled” state, which may be createdwhen the stent 210 is slit along an axis that is substantially parallelto the longitudinal axis and then unrolled. The stent 210 includes acontinuous wave form 212 that includes a plurality of turns 220 that arecreated when the wave form 212 is wrapped around the longitudinal axisLA during manufacturing of the stent 210. The stent 210 generallyincludes a central portion 224 and two end portions, a first end portion226 and a second end portion 228, that are located on opposite sides ofthe central portion 224.

As illustrated in FIG. 3, the wave form 212 includes a plurality ofstruts 230 and a plurality of crowns 232. Each crown 232 is a curvedportion or turn within the wave form 212 that connects adjacent struts230 to define the continuous wave form 212. As shown in FIG. 3, thestruts 230 are substantially straight portions of the wave form 212. Inother embodiments, the struts 230 may be slightly bent or have othershapes, such as a sinusoidal wave, for example.

As illustrated in FIG. 3, the wave form 212 is wrapped around thelongitudinal axis LA at a pitch so that the wave form 212 generallydefines a helical coil in the central portion 224 having a first helicalangle, or first pitch angle α, to define a first helix FH. In theillustrated embodiment, the wave form 212 is also wrapped around thelongitudinal axis LA so the ends of the stent are substantially squareor perpendicular to the longitudinal axis LA. The number of turns 222about the longitudinal axis and the first helical angle α may bedetermined by the particular specifications of the stent 210, such asthe desired unexpanded and expanded diameters and the length of thestent, as well as the size (e.g., diameter) and particular material ofthe wire or strip of material. The illustrated embodiments are notintended to be limiting in any way.

The stent 210 also includes a plurality of connections 240 that areconfigured to connect selected crowns 232 of adjacent turns 222 so thatwhen the stent is in an unexpanded condition, the plurality ofconnections 240 generally lie along a connection helix CH defined by aconnection helical angle β relative to the longitudinal axis LA. Asillustrated in FIG. 3, the connection helix CH is oriented substantiallyopposite to the first helix FH described above such that the connectionhelix CH angle β is between 0° and 90° when using a coordinate systemthat is opposite the coordinate system depicted in FIG. 3 (i.e., thepositive x axis runs from left to right rather than from right to left).

Like the connections 130 discussed above, the connections 240 may becreated by fusing the selected crowns 232 together, as “fusing” isdefined above. In an embodiment, the connections 240 may be created bywelding or soldering the selected crowns 232 together, as “welding” and“soldering” are defined above. In an embodiment, the connections 240 maybe created by fusing, welding, or soldering an additional piece ofmaterial (not shown) that extends between selected crowns 232. Theadditional piece of material may resemble a strut or a portion of astrut, and may be sized to provide spacing between the selected crownsof two adjacent turns, if desired. The illustrated embodiments are notintended to be limiting in any way.

The size of the connections 240 may also be varied according to thedesired flexibility and rate of expansion for a given area of the stent210. In general, the larger the connection 240, i.e. the larger thefusion or weld, the greater the stiffness.

As illustrated in FIG. 3, the struts 230 and the crowns 232 are formedso that the unconnected crowns 232 of adjacent turns 220 that face eachother are spaced from one another so as to form gaps 250 in between thefacing unconnected crowns 232. As illustrated, the gaps 250 aregenerally not uniform and are variable around the circumference of thestent 210, as defined by the pitch angle α, and may also be variablealong the length of the stent 210.

A more detailed view of the end portion 228 of the stent 210 of FIG. 3is illustrated in FIG. 4. As shown, the size of the gaps 250 varybetween the unconnected crowns 232 of adjacent turns 220. For example,for some of the unconnected crowns 232, there is a relatively small gap250 a, and for some of the unconnected crowns 232, there is a relativelylarge gap 250 b. In the illustrated embodiment, there other gaps,represented by 250 c and 250 d, that are in between the small gap 250 aand the large gap 250 b in size. The size of the gap 250 can vary basedon the physical characteristics of the stent, such as strut length,crown design, stent diameter, strut diameter, and other characteristics.In an embodiment, the gap 250 may be in the range from just above zero,i.e., greater than zero, to as high as a distance that is equal to thelength of the longest strut, i.e., less than or equal to the length ofthe longest strut. In an embodiment, the gap 250 may be in the range ofbetween about 0.0005″ and about 0.010″. In an embodiment, the gap 250may be in the range of between about 0.001″ and about 0.003″.

A method that may be used to create the gaps 250 between unconnectedcrowns 232 of adjacent turns 220 is to vary the length of the struts 230and/or size of the crowns 232 in the wave form 212. By varying thelength of the struts 230, the amplitude of the waves of the wave formmay be varied. For example, to increase the gap 250 between unconnectedcrowns 232 of adjacent turns 220, a longer strut 230 a than averageand/or a larger crown than average may be used to form a so-calledextended crown 232. Extended crowns are discussed in further detailbelow with respect to FIGS. 5-7.

Another method that may be used to create the gaps 250 betweenunconnected crowns 232 of adjacent turns 220 is to electro-polish thecrowns 232 after the crowns 232 have been formed. This may be done byelectro-polishing the stent 210 for a pre-determined amount of timeuntil the desire spacing, or gap 250, is achieved between each pair ofunconnected crowns 232. The pre-electro-polished dimensions of thecrowns should be equal to the desired crown dimensions plus the desiredspacing. The desired spacing can vary based on the physicalcharacteristics of the stent, such as strut length, crown design, stentdiameter, strut diameter, and other characteristics. In an embodiment,the spacing may be in the range from just above zero, i.e., greater thanzero, to as high as a distance that is equal to the length of thelongest strut, i.e., less than or equal to the length of the longeststrut. In an embodiment, the spacing may be in the range of betweenabout 0.0005″ and about 0.010″. In an embodiment, the spacing may be inthe range of between about 0.001″ and about 0.003″.

Another method that may be used to create spaces between crowns is tocustomize the kerf of a laser. When laser cutting stents, the width ofthe beam of the laser can be used to create extended crowns. The kerfmay be adjusted by power, speed, and focus of the laser. A wider kerfmay be used just between the crowns and a smaller kerf may be used tocut the remaining parts of the stent. This method may be used inconjunction with electro-polishing.

FIG. 5 illustrates a portion of an embodiment of a stent 410 thatincludes a plurality of turns 420 that are oriented substantiallyperpendicular relative to the longitudinal axis LA of the stent 410.Each turn 420 includes a plurality of struts 430 and a plurality ofcrowns 432. Each crown 432 connects adjacent struts 430 within a turn420 to each other. As illustrated, each turn 420 includes an extendedcrown 432 e that extends into a gap 450 that is defined by the remainingcrowns 432 that face each other. In the embodiment illustrated in FIG.5, the gap 450 is substantially the same around the circumference of thestent 410 and has a length or width of “e” in the Figure. The extendedcrowns may be connected or unconnected in different embodiments. Forexample, in the illustrated embodiment, the extended crowns 432 e arenot connected to each other, but in other embodiments, the extendedcrowns 432 e may be connected to each other.

In accordance with an embodiment of the present invention, a stent 510includes a plurality of turns 520 that are oriented at a pitch angle αrelative to the longitudinal axis LA of the stent 510 to define a firsthelix FH, as illustrated in FIG. 6. Each turn 520 includes a pluralityof struts 530 and a plurality of crowns 532. Each crown 532 connectsadjacent struts 530 within a turn 520 to each other. As illustrated,each turn 520 includes an extended crown 532 e that extends into a gap550 that is defined by the remaining crowns 532 that face each other. Inthe embodiment illustrated in FIG. 6, the gap 550 is substantially thesame around the circumference of the stent 510, as defined by the pitchangle α, and has a length or width “e” in the Figure. Similar to theembodiment of FIG. 5, the extended crowns 532 e are not connected toeach other, but in other embodiments, the extended crowns 532 e may beconnected to each other. In other words, in some embodiments, the crowns532 e are connected and in other embodiments, the crowns 532 e are notconnected.

In the embodiments illustrated in FIGS. 5 and 6, the same spacing isused between the crowns that are not extended. It has been found that insuch embodiments, especially the embodiment illustrated in FIG. 6,non-uniform crimping and/or expansion of the stent may be seen. This maybe due to the longer struts, which are known to bend more easily whensubjected to the same forces as shorter struts having similarcross-sectional dimensions. To improve the uniformity of the crimpingand expansion behavior of the stents of FIGS. 5 and 6, it has been foundthat the amount of extension of the crowns and/or gaps formed betweenthe crowns of adjacent turns may be varied and shifted throughout thestent, i.e., around the circumference and/or along the length of thestent, as illustrated in FIGS. 7 and 8.

FIG. 7 illustrates an embodiment of a stent 610 that includes aplurality of turns 620 that are generally aligned substantiallyperpendicularly to the longitudinal axis of the stent 610. Each turn 620includes a plurality of struts 630 and a plurality of crowns 632, witheach crown 632 connecting adjacent struts 630 within a turn 620 to eachother. As illustrated, each turn 620 includes variable crown extensionsso that two sets of opposing crowns 632 e extend into a gap 650 that isdefined by the remaining crowns 632 that face each other. In theembodiment illustrated in FIG. 7, the gap 650 may be generally the samearound the circumference of the stent 610, but defines a zig-zag-likepattern, as compared to the gap 450 illustrated in FIG. 5. In anembodiment, the gap 650 may be variable around the circumference of thestent 610.

FIG. 8 illustrates an embodiment of a stent 710 that includes a waveform 712 that includes a plurality of turns 720 that are generallyoriented at a pitch angle α relative to the longitudinal axis LA of thestent 710 to define a first helix FH. Each turn 720 includes a pluralityof struts 730 and a plurality of crowns 732, and each crown 732 connectsadjacent struts 730 within a turn 720 to each other. As illustrated,each turn 720 includes an extended crown 732 e that extends into a gap750 that is defined by the remaining crowns 732 that face each other. Inthe embodiment illustrated in FIG. 8, the gap 750 is not constantbetween the turns 720, as illustrated by lines A and B, and is insteadvariable around the circumference of the stent 710 along the pitch angleα.

The variable gap 750 may be created by varying the amplitude of waves ofthe wave form 712 within each of the turns 720 (a wave being defined bytwo adjacent crowns and two adjacent struts connected to the adjacentcrowns). The amplitudes of the waves of the wave form 712 may be variedby varying the lengths of the struts 730 and/or varying the size of thecrowns 732. For example, the outer radius of one crown 732 facing thegap 750 may be larger or smaller than the outer radius of the next crown732 that faces the gap 750 within the same turn 720. The radii of thecrowns 732 may be altered via electro-polishing, as described above. Theradii of the crowns may also be altered during the forming process.Also, other ways of creating/changing the variable spacing include:changing the way that the crowns are connected such as usingbars/bridges between crowns, including bars with sinusoidal shapesbetween crowns, using additional material for welds, or by havingsmaller crown radii on the crowns that are being fused.

Other variations of the embodiments illustrated by FIGS. 7 and 8 may beused to create the desired flexibility, crimping, and expansionproperties of the stent. In addition, embodiments of the variable crownspacing described above may be applied with other design attributes,including but not limited to stent material, strut cross sectiongeometry and dimensions, strut length, crown radius, number of struts inthe cross section of the stent, and number of connection points alongthe stent, to achieve optimal balance between stent deployment symmetry,radial strength upon deployment, and stent flexibility.

The embodiments of the stents discussed above may be formed from a wireor a strip of suitable material. In certain embodiments, the stents maybe formed, i.e., etched or cut, from a thin tube of suitable material,or from a thin plate of suitable material and rolled into a tube.Suitable materials for the stent include but are not limited tostainless steel, iridium, platinum, gold, tungsten, tantalum, palladium,silver, niobium, zirconium, aluminum, copper, indium, ruthenium,molybdenum, niobium, tin, cobalt, nickel, zinc, iron, gallium,manganese, chromium, titanium, aluminum, vanadium, and carbon, as wellas combinations, alloys, and/or laminations thereof. For example, thestent may be formed from a cobalt alloy, such as L605 or MP35N®, Nitinol(nickel-titanium shape memory alloy), ABI (palladium-silver alloy),Elgiloy® (cobalt-chromium-nickel alloy), etc. It is also contemplatedthat the stent may be formed from two or more materials that arelaminated together, such as tantalum that is laminated with MP35N®. Thestents may also be formed from wires having concentric layers ofdifferent metals, alloys, or other materials. Embodiments of the stentmay also be formed from hollow tubes, or tubes that have been filledwith other materials. The aforementioned materials and laminations areintended to be examples and are not intended to be limiting in any way.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient roadmap for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of members described in an exemplary embodimentwithout departing from the scope of the invention as set forth in theappended claims.

1. A stent comprising: a continuous wave form wrapped around alongitudinal axis of the stent at a pitch angle to define a helixcomprising a plurality of turns, the wave form comprising a plurality ofstruts and a plurality of crowns, each crown connecting adjacent strutswithin a turn to define the continuous wave form; and a plurality ofconnections configured to connect selected crowns of adjacent turns,wherein unconnected crowns of adjacent turns that substantially faceeach other are spaced from each other and define a gap therebetween,wherein the gap between the unconnected crowns of adjacent turns isvariable around a circumference of the stent along the pitch angle. 2.The stent according to claim 1, wherein the gap is greater than zero andless than or equal to a length of the longest strut in the wave form. 3.The stent according to claim 2, wherein the gap is between about 0.0005″and about 0.010″.
 4. The stent according to claim 3, wherein the gap isbetween about 0.001″ and about 0.003″.
 5. The stent according to claim1, wherein the connections are fusions of the selected crowns.
 6. Thestent according to claim 1, wherein the connections are welds.
 7. Thestent according to claim 1, wherein the amplitude of the wave formvaries around the circumference of the stent within at least one of theturns.
 8. The stent according to claim 7, wherein the lengths of atleast some of the struts within the at least one of the turns aredifferent.
 9. The stent according to claim 7, wherein the sizes of atleast some of the crowns within the at least one of the turns aredifferent.
 10. A method of manufacturing a stent, the method comprising:forming a wave form comprising a plurality of struts and a plurality ofcrowns, each crown connecting adjacent struts; wrapping the wave formaround a longitudinal axis at a pitch angle relative to the longitudinalaxis to define a helix that includes a plurality of turns substantiallycentered about the longitudinal axis; connecting selected crowns ofadjacent turns; and forming a variable gap between unconnected crowns ofadjacent turns that substantially face each other around a circumferenceof the stent along the pitch angle.
 11. The method according to claim10, wherein the gap is greater than zero and less than or equal to alength of the longest strut in the wave form.
 12. The method accordingto claim 11, wherein the gap is between about 0.0005″ and about 0.010″.13. The method according to claim 12, wherein the gap is between about0.001″ and about 0.003″.
 14. The method according to claim 10, whereinthe connecting comprises fusing the selected crowns to each other. 15.The method according to claim 10, wherein the connecting compriseswelding the selected crowns to each other.
 16. The method according toclaim 10, wherein the forming the gap comprises electro-polishing theunconnected crowns.
 17. The method according to claim 10, wherein theforming the wave form comprises forming extended crowns so that thestruts connected to the extended crowns are longer than an average strutlength of the wave form, and wherein the extended crowns extend into thegap.
 18. The method according to claim 10, wherein the forming the gapoccurs during the wrapping the wave form.